1. Field of the Invention
This invention relates to a reflection-type two-dimensional matrix spatial light modulation device, which is used for a reflection-type flat display device, a video projector, exposure of a photosensitive material, or the like.
2. Description of the Prior Art
Reflection-type two-dimensional matrix spatial light modulation devices have heretofore been utilized in order to constitute reflection-type flat display devices, or the like. A typical one of the reflection-type two-dimensional matrix spatial light modulation devices comprises:
i) a plurality of picture element electrodes, which are located in the form of a two-dimensional matrix made up of rows and columns, PA1 ii) an opposite electrode, which is located at a spacing from the picture element electrodes, and PA1 iii) a light modulation layer, such as a liquid crystal layer, which is located between the opposite electrode and the picture element electrodes, the light modulation layer modulating light, which enters from the side of the opposite electrode into the light modulation layer and is then reflected from the picture element electrodes (or from reflection members located at positions deeper than the picture element electrodes), in accordance with a voltage applied across the light modulation layer by the opposite electrode and each of the picture element electrodes. PA1 i) a plurality of picture element electrodes, which are located in the form of a two-dimensional matrix made up of rows and columns, PA1 ii) an opposite electrode, which is located at a spacing from the picture element electrodes, and PA1 iii) a light modulation layer, which is located between the opposite electrode and the picture element electrodes, the light modulation layer modulating light, which enters from the side of the opposite electrode into the light modulation layer and is then reflected from the picture element electrodes (or from reflection members located at positions deeper than the picture element electrodes), in accordance with a voltage applied across the light modulation layer by the opposite electrode and each of the picture element electrodes, PA1 wherein the improvement comprises the provision of: PA1 a) an insulation film, which is formed between the light modulation layer and the picture element electrodes, and PA1 b) an electrically conductive film, which is formed with the insulation film intervening between the electrically conductive film and the picture element electrodes, the electrically conductive film covering at least a portion of a gap between adjacent picture element electrodes. PA1 i) a plurality of picture element electrodes, which are located in the form of a two-dimensional matrix made up of rows and columns, PA1 ii) an opposite electrode, which is located at a spacing from the picture element electrodes, and PA1 iii) a light modulation layer, which is located between the opposite electrode and the picture element electrodes, the light modulation layer modulating light, which enters from the side of the opposite electrode into the light modulation layer and is then reflected from the picture element electrodes (or from reflection members located at positions deeper than the picture element electrodes), in accordance with a voltage applied across the light modulation layer by the opposite electrode and each of the picture element electrodes, PA1 wherein the improvement comprises the provision of: PA1 a) an insulation film, which is formed on the side of the picture element electrodes opposite to the light modulation layer with the picture element electrodes intervening between the insulation film and the light modulation layer, and PA1 b) an electrically conductive film, which is formed with the insulation film intervening between the electrically conductive film and the picture element electrodes, the electrically conductive film covering at least a portion of a gap between adjacent picture element electrodes, the electrically conductive film being electrically connected to at least one of the picture element electrodes constituting the gap.
As an example, a reflection-type two-dimensional matrix spatial light modulation device, in which a liquid crystal is employed as the light modulation layer, will hereinbelow be described in detail.
Structure of the device
FIG. 1 is a schematic vertical sectional view showing a picture element section of a conventional reflection-type two-dimensional matrix spatial light modulation device. As illustrated in FIG. 1, an n-MOS-FET 11 and a charge storage capacity Cstg 12 are formed on a single crystalline p.sup.- -type silicon semiconductor substrate 10. The n-MOS-FET 11 comprises an n.sup.+ -type drain region 13, an n.sup.+ -type source region 14, a gate oxide film 15, and a gate electrode 16 constituted of a poly-Si film. The charge storage capacity Cstg 12 comprises a p+region 17, an oxide film 18, and a poly-Si film 19.
A first-layer Al wiring 21 is formed on the side across a first layer-insulation film 20, and a source electrode 22 connected to the source region 14 is thereby formed. The source region 14 and the poly-Si film 19 of the charge storage capacity Cstg 12 are connected to each other by the source electrode 22. A drain electrode 23 is connected to the drain region 13. Also, a picture element electrode (second-layer Al) 25 is formed on the side across a second layer-insulation film 24 and is connected to the source electrode 22.
An orientation film 26 is formed on the picture element electrode 25. An opposite transparent common electrode 28 constituted of ITO is formed on one side of an opposite transparent substrate 27, and an orientation film 29 is formed on the opposite transparent common electrode 28. The two substrates 10 and 27 are located such that the orientation film 26, which is combined integrally with the substrate 10, and the orientation film 29, which is combined integrally with the substrate 27, may stand facing each other. A liquid crystal 30 is held between the orientation film 26 and the orientation film 29.
FIG. 2 is a circuit diagram showing a circuit equivalent to the picture element section of the spatial light modulation device shown in FIG. 1. As illustrated in FIG. 2, the source electrode 22 of the n-MOS-FET 11, one side of the charge storage capacity Cstg 12, and the picture element electrode 25 are connected to one another.
The other side of the charge storage capacity Cstg 12 is connected to a power source ground potential Vss of the spatial light modulation device. Also, a capacity Clc is formed by the picture element electrode 25, the orientation films 26, 29, the liquid crystal 30, and the opposite transparent common electrode 28.
The gate electrode voltage of the n-MOS-FET 11, which is taken with respect to the power source ground potential Vss, is represented by Vg, and the drain electrode voltage, which is taken with respect to the power source ground potential Vss, is represented by Vd. The source electrode voltage, which is taken with respect to the power source ground potential Vss, is represented by Vs, and the opposite transparent common electrode voltage, which is taken with respect to the power source ground potential Vss, is represented by Vcom. Also, the picture element electrode voltage, which is taken with respect to Vcom, is represented as a liquid crystal layer voltage Vlc.
Fundamental operation of the device
There are various kinds of liquid crystals, which serve as light modulation materials, and various electro-optic modes of them. Several examples will be described hereinbelow.
(1) Example using a ferroelectric liquid crystal
FIG. 3 is a schematic view showing a light modulating optical system utilizing the spatial light modulation device shown in FIG. 1, the view serving as an aid in explaining a fundamental operation of the spatial light modulation device. As illustrated in FIG. 3, a polarization beam splitter (hereinbelow referred to as the PBS) 6 is located on the opposite transparent substrate side of a spatial light modulation device 5. Light is produced by a light source 7 and is irradiated to the PBS 6. As a result, the S-polarized wave is reflected by the PBS 6 and impinges upon the opposite transparent substrate 27 of the spatial light modulation device 5. The incident light passes through the layer of the liquid crystal 30 and is reflected from the picture element electrode 25. The reflected light passes through the liquid crystal layer and impinges upon the PBS 6. At this time, only the P-polarized wave component of the reflected light passes through the PBS 6 and is thereby obtained as the output light.
FIG. 4 is an explanatory view showing the relationship between the liquid crystal layer voltage Vlc and the position of orientation of the liquid crystal, the view serving as an aid in explaining the fundamental operation of the spatial light modulation device shown in FIG. 1. In this example, as the liquid crystal, a ferroelectric liquid crystal exhibiting bistable orientation is used. As illustrated in FIG. 4, orientation processing is carried out such that, when the liquid crystal layer voltage Vlc is equal to -Vlcs, the direction along which the liquid crystal is orientated may coincide with the incident polarization axis, and such that, when the liquid crystal layer voltage Vlc is equal to Vlcs, the direction along which the liquid crystal is orientated may make an angle of 45 degrees with respect to the incident polarization axis. Also, the material of the liquid crystal and the thickness of the liquid crystal layer are adjusted appropriately such that, the direction along which the liquid crystal is orientated may make an angle of 45 degrees with respect to the incident polarization axis, the desired output light may be obtained.
In this manner, the output light goes to the off level when the liquid crystal layer voltage Vlc is equal to -Vlcs. Also, the output light goes to the on level when the liquid crystal layer voltage Vlc is equal to Vlcs.
FIG. 5 is a graph showing the voltages at the picture element section in the constitution described above with reference to FIGS. 1 through 4, and the wave form of output light. With reference to FIG. 5, firstly, such that the n-MOS-FET 11 may be triggered into a conducting state, the gate electrode voltage Vg is set at Vgon, which is of a sufficiently high level. At the same time, the drain electrode voltage Vd is set at Vd(on). As a result, the picture element voltage Vs becomes approximately equal to Vd(on). Thereafter, even if the gate electrode voltage Vg is set at Vgoff, which is of a sufficiently low level, such that the n-MOS-FET 11 may go to a non-conducting state, the picture element voltage Vs will be kept at approximately Vd(on) by the effects of the charge storage capacity Cstg 12 and the liquid crystal layer capacity Clc. Therefore, during this period as indicated by (a) in FIG. 5, the liquid crystal layer voltage Vlc is represented by the formula Vlc=(Vd(on)-Vcom).
Also, when the gate electrode voltage Vg is set to be sufficiently high such that the n-MOS-FET 11 may be triggered into a conducting state, and at the same time the drain electrode voltage Vd is set at Vd(off), the picture element voltage Vs becomes approximately equal to Vd(off). Thereafter, even if the gate electrode voltage Vg is set to be sufficiently low such that the n-MOS-FET 11 may go to a non-conducting state, the picture element voltage Vs will be kept at approximately Vd(off) by the effects of the charge storage capacity Cstg 12 and the liquid crystal layer capacity Clc. Therefore, during this period as indicated by (b) in FIG. 5, the liquid crystal layer voltage Vlc is represented approximately by the formula Vlc=(Vd(off)-Vcom).
In cases where the opposite transparent common electrode voltage Vcom is applied such that EQU Vcom=(Vd(on)+Vd(off))/2
the liquid crystal layer voltage Vlc during the period (a) and the liquid crystal layer voltage Vlc during the period (b) are represented by the formulas shown below.
Period (a): EQU Vlc=(Vd(on)-Vd(off))/2
Period (b): EQU Vlc=-(Vd(on)-Vd(off))/2
In such cases, Vd(on) and Vd(off) may be determined such that the liquid crystal layer voltage Vlc during the period (a) may be equal to at least Vlcs and such that the liquid crystal layer voltage Vlc during the period (b) may be equal to at most -Vlcs. In this manner, the output light can be modulated such that it may be on during the period (a) and may be off during the period (b).
Actually, due to a parasitic capacity of the n-MOS-FET 11, or the like, it often occurs that the liquid crystal layer voltage Vlc during the period (a) and the liquid crystal layer voltage Vlc during the period (b) are not symmetric with respect to each other. In such cases, Vcom is adjusted such that the DC component may become zero.
(2) Example using an electrically controlled birefringence (ECB) mode liquid crystal
In this example, as in FIG. 3, the PBS is utilized in the light modulating optical system.
FIG. 6 is a schematic plan view showing a liquid crystal orientation state in an ECB mode liquid crystal layer in accordance with the liquid crystal layer voltage Vlc. FIG. 7 is a schematic side view showing the liquid crystal orientation state in the ECB mode liquid crystal layer in accordance with the liquid crystal layer voltage Vlc. As the liquid crystal, an ECB mode liquid crystal is used, which has a negative dielectric constant anisotropy, which exhibits birefringence with the minor axis and the major axis of the molecule, and which operates with an AC voltage. As illustrated in FIGS. 6 and 7, when Vlc=0, the liquid crystal molecule is orientated vertically. When the AC voltage Vlc becomes high, the liquid crystal molecule inclines so as to become parallel to the electrode surface. The orientation processing is carried out such that the direction of inclination of the liquid crystal molecule may make an angle of approximately 45.degree. with respect to the incident polarization axis.
FIG. 8 is a graph showing dependency of reflected output light, which is obtained from the ECB mode liquid crystal layer, upon the liquid crystal layer voltage.
As illustrated in FIG. 8, when Vlc=0, the apparent birefringence, as viewed from the incident light side, is small, and the intensity of the reflected output light takes the lowest value. When Vlc is increased and goes beyond a threshold value voltage Vlc(th), the liquid crystal molecule begins inclining, the apparent birefringence becomes large, and the intensity of the reflected output light becomes high. At the time at which Vlc=Vlcs, the intensity of the reflected output light becomes highest.
Therefore, the reflected output light can be modulated by changing Vlc from Vlc(th) to Vlcs.
(3) Example using a polymer-dispersed liquid crystal
In this example, a Schlieren optical system is utilized in the light modulating optical system. The Schlieren optical system transmits non-scattered light and eliminates scattered light.
In an example of a polymer-dispersed liquid crystal, a liquid crystal having a positive dielectric constant anisotropy and operating with an AC voltage is dispersed in a polymer network. In such a polymer-dispersed liquid crystal, when Vlc=0, the liquid crystal molecule is regulated by the interface of the polymer and becomes orientated at random. When Vlc is increased, the liquid crystal molecule is orientated such that its major axis may be perpendicular to the electrode surface. In accordance with the relationship between the birefringence (ne))no) exhibited by the liquid crystal and the refractive index np (approximately equal to no) of the polymer, when the liquid crystal is orientated at random, the reflected light is scattered. Also, as the direction, along which the liquid crystal is orientated, becomes perpendicular to the electrode surface, the reflected light becomes non-scattered light. Therefore, the reflected light is scattered when Vlc=0, and becomes non-scattered light when Vlc is increased.
In cases where such reflected light is projected through the Schlieren optical system, the output light is obtained, which exhibits the dependency upon the voltage (AC voltage) as shown in FIG. 9. Specifically, when Vlc=0, the intensity of the output light takes the lowest value. When Vlc is increased and goes beyond a threshold value voltage Vlc(th), the intensity of the output light becomes high. At the time at which Vlc is equal to at least Vlcs, the intensity of the reflected output light becomes highest. Therefore, the reflected output light can be modulated by changing Vlc from Vlc(th) to Vics.
(4) Example using a guest host liquid crystal (positive type)
In this example, it is unnecessary to use a particular light modulating optical system, and only a projection lens may be utilized.
An example of a guest host liquid crystal (positive type) comprises a cholesteric-nematic phase transition type of liquid crystal, which has a positive dielectric constant anisotropy and operates with an AC voltage, and a guest (a dichroic dye) added to the liquid crystal. When Vlc=0, the liquid crystal molecule takes a spiral form, the incident light is absorbed by the guest, and the intensity of the reflected output light takes the lowest value. When Vlc is increased, the spiral structure of the liquid crystal molecule is released, the liquid crystal molecule is orientated such that its major axis may be perpendicular to the electrode surface, and the guest stands in parallel with the liquid crystal molecule. Therefore, the degree of absorption of the incident light becomes low, and the intensity of the reflected output light becomes high.
FIG. 10 shows the dependency of the output light, which is obtained from the positive type of guest host liquid crystal layer, upon the voltage (AC voltage). As illustrated in FIG. 10, when Vlc=0, the intensity of the output light takes the lowest value. When Vlc is increased and goes beyond a threshold value voltage Vlc(th), the intensity of the output light becomes high. At the time at which Vlc is equal to at least Vlcs, the intensity of the reflected output light becomes highest. Therefore, the reflected output light can be modulated by changing Vlc from Vlc(th) to Vlcs.
(5) Example using a guest host liquid crystal (negative type)
In this example, as in the example described in (4), it is unnecessary to use a particular light modulating optical system, and only a projection lens may be utilized.
An example of a guest host liquid crystal (negative type) comprises a cholesteric-nematic phase transition type of liquid crystal, which has a negative dielectric constant anisotropy and operates with an AC voltage, and a guest (a dichroic dye) added to the liquid crystal. When Vlc=0, the liquid crystal molecule is orientated perpendicularly to the electrode surface, the degree of absorption of the incident light is low, and the intensity of the reflected output light takes the highest value. When Vlc is increased, the liquid crystal molecule takes a spiral form, the degree of absorption of the incident light becomes high, and the intensity of the reflected output light becomes low.
FIG. 11 shows the dependency of the output light, which is obtained from the negative type of guest host liquid crystal layer, upon the voltage (AC voltage). As illustrated in FIG. 11, when Vlc=0, the intensity of the output light takes the highest value. When Vlc is increased and goes beyond a threshold value voltage Vlc(th), the intensity of the output light becomes low. At the time at which Vlc is equal to at least Vlcs, the intensity of the reflected output light takes the lowest value. Therefore, the reflected output light can be modulated by changing Vlc from Vlc(th) to Vlcs.
As described above, the reflected output light can be modulated by the utilization of various operation modes of liquid crystals. The operations are those of the liquid crystal held between the opposite transparent common electrode and each of the picture element electrodes. However, as for the operation of the liquid crystal at the portion, which stands facing a gap between adjacent picture element electrodes, the problems often occur that the operation becomes indeterminate. The problems will be described hereinbelow.
FIG. 12 is an explanatory sectional view showing a region, which stands facing a picture element electrode, and a region, which stands facing a gap between adjacent picture element electrodes, in a conventional reflection-type two-dimensional matrix spatial light modulation device. With reference to FIG. 12, the liquid crystal layer voltage at the region, which stands facing a picture element electrode, is represented by Vlcp (=Vs-Vcom), and the liquid crystal layer voltage at the region, which stands facing a gap between adjacent picture element electrodes, is represented by Vlcm (which is taken with respect to Vcom). Also, the reflected output light obtained from the region, which stands facing the picture element electrode, is represented by Rp, and the reflected output light obtained from the region, which stands facing the gap between the adjacent picture element electrodes, is represented by Rm. As for the region, which stands facing the picture element electrode, Vlcp is determinate, and the output light undergoes the operations described above. However, as for the region, which stands facing the gap between the adjacent picture element electrodes, the liquid crystal layer potential with respect to Vcom is affected by potentials of the substrate, the picture element circuit, and the like, which are located below the gap between the adjacent picture element electrodes, and the electric fields coming from the adjacent picture element electrodes. As a result, Vlcm becomes indeterminate, and the reflected output light obtained from the region, which stands facing the gap between the adjacent picture element electrodes, becomes indefinite.
Specifically, in the aforesaid example using the ferroelectric liquid crystal, when the liquid crystal layer potential with respect to Vcom is higher than Vcom, the liquid crystal layer voltage Vlcm becomes positive, and the output light goes to the on level in accordance with FIG. 5.
In the aforesaid example using the ECB mode liquid crystal, the polymer-dispersed liquid crystal, or the guest host liquid crystal (positive type), when the liquid crystal layer voltage Vlcm at the region, which stands facing the gap between the adjacent picture element electrodes, shown in FIG. 12 is higher than Vlc(th), the intensity of the output light becomes higher than the of f level in accordance with FIG. 8, FIG. 9, or FIG. 10.
In the aforesaid example using the guest host liquid crystal (negative type), when the liquid crystal layer voltage Vlcm at the region, which stands facing the gap between the adjacent picture element electrodes, shown in FIG. 12 is lower than Vlcs, the intensity of the output light becomes higher than the off level in accordance with FIG. 11.
The foregoing means that unnecessary light is radiated out from the region, which stands facing the gap between the adjacent picture element electrodes. If such problems occur, in cases where the spatial light modulation device is utilized for a projector, the contrast will become low. Also, in cases where the spatial light modulation device is utilized for the exposure of a photosensitive material, the image quality will be affected adversely.
Further, if the light incident upon the spatial light modulation device leaks through the gap between the adjacent picture element electrodes to the semiconductor constituting the picture element circuit, the source potential will often be fluctuated due to optically pumped carrier. Fluctuations in the source potential also cause the image quality to become bad.
In cases where picture elements of a finer resolution are set, the device size is reduced even further, and the picture element size is thereby rendered smaller, the problems described above occur more markedly.
As a countermeasure for the problems described above, it has been proposed to locate an electrically conductive or non-conductive light blocking film (a light absorbing layer or a light reflecting layer) above a transistor, above a wiring, or between a picture element electrode and a picture element circuit. Such a countermeasure is proposed in, for example, Japanese Patent Publication Nos. 57(1982)-39422 and 61(1986)-43712. Also, it has been proposed to locate a dielectric multi-layer film on a transistor or on a picture element electrode and thereby to reflect incident light at a gap between picture elements. Such a countermeasure is proposed in, for example, Japanese Patent Publication No. 4(1992)-51070 and Japanese Unexamined Patent Publication No. 4(1992)-338721.
However, with the proposed countermeasures, even though the effects of reducing the light leaking to the semiconductor can be obtained, the requirements with regard to the image quality with the output light and the light utilization efficiency cannot be satisfied sufficiently.
FIG. 13 is a schematic view showing a distribution of intensity of output light obtained from various regions in a conventional reflection-type two-dimensional matrix spatial light modulation device, in which a light blocking layer is a light absorbing layer. As illustrated in FIG. 13, the intensity of the reflected output light, which is obtained from the region facing the gap between the adjacent picture elements, is always low, and therefore the light utilization efficiency cannot be kept high. Also, the portions, at which the intensity of the reflected output light is low, constitute a lattice-like black matrix. Accordingly, in cases where the spatial light modulation device is utilized for a projector, interference with a lattice-like optical element, such as a lenticular lens screen, is caused to occur, and the image quality cannot be kept good. Further, in cases where the spatial light modulation device is utilized for exposure of a negative photosensitive material, the region, which stands facing the gap between the adjacent picture elements, always constitutes a highlight in the obtained image, and therefore the contrast of the image becomes markedly low.
FIG. 14 is a schematic view showing a distribution of intensity of output light obtained from various regions in a different conventional reflection-type two-dimensional matrix spatial light modulation device, in which a light blocking layer is a light reflecting layer. As described above, the liquid crystal layer voltage at the region, which stands facing the gap between the adjacent picture elements, is unstable. Therefore, as illustrated in FIG. 14, the reflected output light Rm, which is obtained from the region facing the gap between the adjacent picture elements, becomes unstable. As a result, the image quality cannot be kept high.
In order for the aforesaid problems to be eliminated, there has also been proposed a countermeasure, in which electrically conductive light blocking layers are connected to an electric power source Vm that is common to the device, potentials of the electrically conductive light blocking layers are controlled, and the liquid crystal layer voltage at the region facing the gap between the adjacent picture elements is thereby stabilized. FIG. 15 is a schematic view showing a distribution of intensity of output light obtained from various regions in a further different conventional reflection-type two-dimensional matrix spatial light modulation device, in which electrically conductive light blocking layers are utilized. As illustrated in FIG. 15, with the proposed countermeasure, even though the liquid crystal layer voltage at the region, which stands facing the gap between the adjacent picture elements, becomes stable, since the electrically conductive light blocking layers are controlled with the voltage common to the device, the voltage always takes a fixed value regardless of the liquid crystal layer voltage at the region above each of the picture element electrodes. Therefore, it cannot be said that the proposed countermeasure is a drastic countermeasure for the aforesaid problems. Further, with the proposed countermeasure, it is necessary to lay a particular wiring for connecting the light blocking layers to the electric power source Vm, and problems occur in that the production yield and the reliability cannot be kept high due to an increase in the number of processes, breakage of the wiring, and short-circuiting.